Steering mode selection valves are used in hydrostatic vehicles with multi-axle steering, preferably vehicles with two steering axles such as mobile dredgers, street-cleaning, equipment, rotary snowplows, rollers, and the like. The steering mode selection valves are used to select the desired steering mode, and in particular to make a selection between front-axle steering, rear-axle steering, two-axle steering, and crab steering.
Such steering mode selection valves are known in the most variant forms. For instance, a steering mode selection valve may be embodied as a hand-actuated rotary slide valve, however, this version has the disadvantage that because of the functionally dictated play between the rotary slide and the valve housing, the pressure medium leaks, and this leakage can hardly be reduced to desired values at feasible expense. Moreover, as a rule there is a demand in a steering mode selection valve to allow the selection valve to be mounted in the driver's cab, to assure convenient operation of the vehicle. In hand-actuated rotary slide valves, this requires a complicated course of the lines, because it is understood that the pressure medium must be brought as far as the valve.
Electrically actuatable slide valves are also known which while they do avoid this last disadvantage of hand-actuated valves nevertheless still have the disadvantage of leakage. Known electrically actuated slide valves are often embodied as detent-locked slide valves. These valves have a magnet-actuated piston which can be switched from a middle position into two further switching positions, and the piston locks in these two further positions in detent fashion so that the actuation forces need not be maintained constantly. In such steering mode selection valves, only three steering modes can therefore be selected.
It is moreover known to construct a steering mode selection valve from a plurality of position-seat valves which communicate with one another over corresponding lines. By using seat valves, the disadvantage of leakage as in slide valves is avoided. Since in valves embodied in this way it is undesirable for a human operator to have to actuate a plurality of valves in order to select one steering mode, these steering mode selection valves in principle have electrically actuatable position-seat valves, which can be triggered in the desired way from the control unit. However, this dictates the disadvantage that if an electrical actuating arrangement is defective or if there is a voltage failure, valve positions can arise that are not immedately definable and are extremely dangerous. This can lead to sudden establishment of the wrong steering mode, where the operator does not notice this until the vehicle reacts in an unexpected way. Hence in such cases there is a direct threat to the vehicle, the operator, and the surroundings of the vehicle.